The present invention relates to a flowpath configuration for a turbine facilitating streamline flow characteristics along the flowpath and sealing and particularly relates to a flowpath configuration for a steam turbine for minimizing leakage flow and secondary aerodynamic losses at the steam path root regions.
The flowpath through a turbine along the root radius is defined in part by the inner bands or rings of the nozzles and flow surfaces along the platforms at the roots of the buckets on the rotor. Any fluid flow leakage exiting the flowpath along the root radii bypasses the buckets and directly decreases the power output of the turbine stage. A typical nozzle and bucket design, for example, for a low pressure section of a steam turbine, includes a nozzle root diameter equal to the bucket root diameter, resulting in a significant probability of an upstream facing step at a steady state flow condition which disturbs the streamline characteristics of the fluid flow in the flowpath. Large wheel spaces also increase the rotor pumping effect of leakage flows and therefore increase radial intrusion flow which causes further aerodynamic losses. More particular, radial reentry flows caused by rotor pumping effects cause fluid flow separation along the flowpath with consequent aerodynamic efficiency losses. Accordingly, there has developed a need for a root radius flowpath configuration for a turbine which will ensure that streamlining of fluid flow in the flowpath is substantially independent of flowpath degradation by minimizing leakage flow and secondary aerodynamic losses in the fluid flowpath root region.
In accordance with a preferred embodiment of the present invention, there is provided a flowpath root region which substantially minimizes upset of the fluid flow in the flowpath, minimizes leakage flow and facilitates streamline flow in the flowpath. Particularly, the root region of the flowpath includes the inner band of the nozzles and the surfaces of the platforms at the roots of the buckets. The bucket platforms form part of the bucket dovetails. Each bucket dovetail includes entrance and exit root side radial seals radially inwardly of the platforms and radially underlying exit and entrance labyrinth seals on adjacent nozzles. These seals reduce leakage flows into and out of the wheel spaces between the rotor wheel and adjacent nozzles. The wheel spaces between the dovetails and rotor wheel, on the one hand, and the nozzles on the other hand, are reduced to reduce the rotor pumping action and, hence, intrusion flow returning to the flowpath.
It will be appreciated that combined leakage flows pass between the nozzles and the buckets for entry into the upstream wheel space where the flow combines with leakage flow through an upstream packing ring for passage through a wheel hole to a downstream wheel space. The leakage flow into the downstream wheel space in part exits into the fluid flowpath past the exit root radial seal. There is provided adjacent the exit root radial seal an exit flow guide which minimizes flowpath disturbance by decreasing the radial component of the intrusion flow, i.e., the return leakage flow into the flowpath has a substantially large or predominant axial flow component, as compared with its radial flow component. The predominant axial flow component minimizes upset of the fluid flow in the flowpath. The exit flow guide becomes increasingly important as packing seal capability decreases with time, causing greater intrusion flows returning to the fluid flowpath. The exit flow guide also serves to minimize the axial distance between the bucket and the next stage nozzle, facilitating flow streamlining in the flowpath.
Each bucket also has an entrance root radius extending axially upstream and radially inwardly to minimize or eliminate any flowpath entrance projection in the path of the fluid exiting the trailing edge of the inner band of the upstream nozzle. This minimizes the possibility of an axially forwardly facing step at steady state condition where such step could interrupt the fluid flow in the flowpath. Thus, the bucket entrance root diameter on its upstream side is less than the nozzle exit root diameter on the downstream side. Similarly, the downstream nozzle entrance root radius lies radially inwardly of the trailing edge of the upstream platform surface. This likewise avoids upsets in the fluid flowing along the flowpath and affords a robustness between the bucket exit and nozzle entrance.
Additionally, on the leading edge of the bucket platform, there is provided an entrance root axial sealing fin which affords an additional reduction in flow coefficient, further reducing leakage flow. The axial sealing fin also reduces the axial distance between the nozzle and bucket to improve fluid path streamline characteristics in the flowpath.
In a preferred embodiment according to the present invention, there is provided a turbine comprising a rotor having wheels at axially spaced locations along the rotor and mounting a plurality of circumferentially spaced buckets, the rotor being rotatable about an axis, axially spaced circumferential arrays of nozzles having circumferentially spaced airfoils and inner and outer bands at opposite ends thereof, the axially spaced buckets and the arrays of nozzles forming at least a pair of axially spaced stages of the turbine, the buckets having dovetails for securing the buckets to the rotor wheels and platforms along radially inner ends of the buckets, the platforms, the airfoils, the inner and outer bands and the buckets in part defining a flowpath for fluid flow through the turbine, the bucket dovetails on one of the wheels mounting projections extending generally axially toward one of the arrays of nozzles along locations radially inwardly of the platforms, the nozzles of the one array thereof carrying labyrinth teeth forming with the projections a seal to reduce leakage flow from the flowpath into a wheel space between the one wheel and the one array of nozzles.
In a further preferred embodiment according to the present invention, there is provided a flowpath streamlining configuration for root regions of a turbine flowpath comprising a rotor rotatable about an axis and mounting a plurality of circumferentially spaced buckets, an axially spaced circumferential array of nozzles having circumferentially spaced airfoils with inner and outer bands at opposite ends thereof spaced axially downstream of the buckets, the buckets having dovetails for securing the buckets and rotor to one another and platforms along radially inner ends thereof, the platforms and the inner bands in part defining the root region of the flowpath for fluid flow through the turbine, the bucket dovetails including exit flow guides along a downstream side of the dovetails for directing leakage fluid flow from a wheel space between the dovetails and the nozzles into the flowpath in a predominantly downstream axial direction.
In a further preferred embodiment according to the present invention, there is provided a turbine comprising a rotor rotatable about an axis and mounting a plurality of circumferentially spaced buckets having platforms along radially inner ends thereof, an axially spaced circumferential array of nozzles having circumferentially spaced airfoils with inner and outer bands at opposite ends thereof, the platforms, the buckets, the inner and outer bands and the airfoils in part defining a flowpath for fluid flow through the turbine, the array of nozzles being axially spaced upstream of the buckets and leading edges of the bucket platforms lying radially inwardly of trailing edges of the upstream array of nozzles.